Microwave heating is a well known technique whereby microwave frequencies, typically greater than 500 Megahertz, are applied through an applicator, including cavities and waveguides, to heat a variety of materials and/or objects. While microwave energy can be generated by a number of devices, for instance, the klystron, the traveling wave tube, and the magnetron, the use of the magnetron in heating applications is widely known. The magnetron is, however, a device which can only be operated efficiently under certain operating conditions based on known structural characteristics.
The magnetron typically consists of a hollow copper anode having a resonant microwave structure and an electron emitting cathode located at the center thereof. Electrons are emitted from the cathode and attracted to the anode if the anode is positively charged relative to the cathode. When electrons are attracted from the cathode towards the anode, a magnetic field around the cavity and the applied electric field cause electrons to travel in a path about the cathode. The anode, which includes a number of cavities, has one cavity used to direct the developed microwave energy towards an attached applicator, such as the waveguide.
When starting a magnetron, a no load condition occurs. At startup, no current or electrons flow between the cathode and anode until a specific voltage is reached, called the .PI.-mode (pi mode) voltage. Once reached, anode current rises rapidly reaching its maximum rated value with a further voltage increase of only a small amount. Since the magnetron includes a number of cavities tightly coupled together, a number of other possible field distributions, called modes, including the .PI.-mode, are possible. Some of those modes may be close to each other in frequency. The .PI.-mode is, however, the most efficient of all the modes, and, consequently, the magnetron operates most efficiently in this mode. Unwanted modes, on the other hand, resonate at incorrect frequencies, also known as moding, wherein the magnetron efficiency is low. Excessive internal heating can occur and damage the magnetron at the incorrect frequencies.
A power supply drives the magnetron and is an important part of any microwave circuit, since the output frequency of the magnetron depends, in part, upon the power supply itself and the applicator to which the magnetron is connected. For instance, in a microwave oven, for cooking or thawing foods, the power output of the magnetron typically ranges anywhere from zero to 1,500 watts depending on the type of foods being prepared. In addition, because foods can be cooked with a relatively wide frequency range of microwaves, the power supplies for such microwave ovens are not generally directed towards accurate control of the output frequency of the magnetron. In certain industrial processes, however, the power level and frequency range is more tightly controlled due to the nature of the material and/or process being performed. Consequently, various methods and apparatus are known for supplying power to a magnetron to cause the magnetron to mode in the proper frequency and to generate the necessary output power. The following references describe these and other methods and apparatus for supplying power to a magnetron.
In U.S. Pat. No. 3,651,371 to Tingley, a power supply for a magnetron and a microwave oven is described. A high impedance transformer furnishes power to half-wave, oppositely pulled, voltage doubler circuits in which a time delay is provided responsive to the load current of the magnetron, to delay the turning on of one of the half-wave voltage doubler circuits to insure operation in the desired oscillating mode. The filament of the magnetron is fed by a separate filament transformer turned on at the same time as the high impedance transformer but which includes means for lowering the filament voltage incident to switching to the high power mode.
U.S. Pat. No. 3,873,883 to Seivers et al., describes a positive ignition power supply for a magnetron. The power supply includes a step-up transformer having a primary winding for connecting to an AC power source and at least one secondary winding. A full wave voltage multiplying rectifier circuit connected to the secondary winding and the anode-cathode circuit of the magnetron applies a time varying voltage across the anode-cathode circuit of the magnetron. The filament circuit of the magnetron and the anode-cathode circuit are simultaneously energized. The time varying voltage applied to the anode-cathode circuit insures that the magnetron goes into a proper mode of oscillation.
U.S. Pat. No. 4,481,447 to Stupp et al., describes a method of controlling the power output of a magnetron and an electric power supply for supplying power to the magnetron. Power is continuously supplied to the magnetron heater while at the same time, a voltage is continuously applied across the anode and cathode of the magnetron. The voltage across the anode and the cathode repeatedly varies in cycles between a first value, which is substantially at or below the threshold voltage of the magnetron tube, and a second value, which is above the threshold voltage.
U.S. Pat. No. 4,742,442 to Nilssen, describes a power supply for a magnetron in a microwave oven. A full bridge inverter power supply includes two pairs of switching transistors and is conditionally operable to self oscillate in one of two modes. In the first mode, one of the two pairs of switching transistors self oscillates in the manner of a half bridge inverter and powers the cathode. In the second mode, both pairs of transistors self oscillate in the manner of a full bridge inverter and provide the anode power as well as heating power.
U.S. Pat. No. 5,003,141 to Braunisch et al., describes a magnetron power supply with indirect sensing of magnetron current. A switch mode power supply, which drives the magnetron, includes a resonance circuit having a transformer connected to the magnetron by a multiplier consisting of a rectifier and voltage doubler circuit. A current transformer is connected in series with one of the diodes in the rectifier and voltage doubler circuit to obtain a feedback signal which is proportional to the power fed to the magnetron. The sensed feedback signal is compared in a control circuit with a reference signal, the comparison of which is used to control the switch frequency and thereby the magnetron power.
U.S. Pat. No. 5,082,998 to Yoshioka, describes a switching power supply in which DC power is changed to a pulse by means of a switching element coupled to a primary winding of an inverter transformer to supply power from a secondary winding of the transformer to a magnetron coupled thereto. The inverter transformer has a supplementary winding which is coupled to the control side of the switching element to form a self-excited voltage resonance type.
U.S. Pat. No. 5,224,027 to Kyong-keun, describes a power supply for a magnetron wherein as abrupt current changes occur under loaded power supplies, the power supply detects currents and protects the magnetron from overcurrents by controlling output voltages through feeding back the voltages according to currents and by outputting stable power supplies.
U.S. Pat. No. 5,250,774 to Lee, describes a power supply circuit for driving a magnetron equipped in a microwave oven which provides a stable power to the magnetron by preventing instability of output voltage due to LC resonance between a high voltage condenser and by good insulation between the secondary windings of the transformer in a switching mode power supply employing pulse width modulation.